Implementing pinch-zoom and pan/drag in an Android view on the canvas

@Override
public boolean onScale(ScaleGestureDetector detector) {
    scaleFactor *= detector.getScaleFactor();
    scaleFactor = Math.max(MIN_ZOOM, Math.min(scaleFactor, MAX_ZOOM));
    return true;
}

@Override
public boolean onTouchEvent(MotionEvent event) {

    switch (event.getAction() & MotionEvent.ACTION_MASK) {
        ...
        ...
    }

    //This will set the value of scaleFactor
    detector.onTouchEvent(event);

    //The only time we want to re-draw the canvas is if we are panning (which happens when the mode is
    //DRAG and the zoom factor is not equal to 1) or if we're zooming
    if ((mode == DRAG && scaleFactor != 1f) || mode == ZOOM) {
        invalidate();
    }

    return true;
}

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.save();

    //We're going to scale the X and Y coordinates by the same amount
    canvas.scale(scaleFactor, scaleFactor);

    //We need to divide by the scale factor here, otherwise we end up with excessive panning based on our zoom level
    //because the translation amount also gets scaled according to how much we've zoomed into the canvas.
    canvas.translate(translateX / scaleFactor, translateY / scaleFactor);
    canvas.restore();
}